This invention relates to control mechanisms for drive trains of vehicles and more particularly relates to manual bypass mechanisms for electronically controlled fuel injection systems in military vehicles such as M1 or M1A1 Tanks.
The U.S. Army now uses a fuel injection device known as an HMU or hydromechanical unit, on its M1 and M1A1 Tanks. The HMU is an assembly of electrically actuatable valves and other components which are controlled by an ECU (electronic control unit). To prevent engine damage, the ECU is programmed to initiate several protective modes and reduces fuel flow from the HMU if certain performance irregularities occur in the engine, transmission, or other propulsion system component. In one of the protective modes, referred to as "protective mode III", the flow rate of fuel to the engine is reduced to about 120 lbs./hour, so the engine generates only about 72 hp, or 5 percent of its capacity. The tank must move very slowly, at 1 mph, to a site where the propulsion system's problem can be analyzed.
In a battle or emergency scenario, it may be necessary to drive the tank faster than protective mode III permits. Further, it is possible that battle damage could cut off electrical power to the ECU and thereby adversely limit fuel flow to the engine.
To address these difficulties, we have created a completely mechanical fuel bypass device which routes additional fuel to the engine during protective mode III or during failure of the ECU. The device is a modular, add-on unit that can be installed on existing M1 or M1A1 tanks without disassembly of the hydromechanical units and which can be used without modifying the logic of the ECUs. The device has a screw-type metering valve which is highly accurate even though its metering shaft moves only a quarter turn between its zero-flow position to its fully open position. When our fuel bypass device is fully open, it permits a fuel flow of approximately 230 lbs/hr which when combined with fuel flow from the HMU, will produce about 400 hp, or nearly 30% of the engine's power. Consequently, the tank can travel out of danger 10 to 15 times faster than it ordinarily could during a protective mode III condition. Additionally, only a small movement of the mechanical linkage for the valve is needed to operate it, so the valve and linkage use relatively little of the limited space in the engine compartment.